1. Field of the Invention
This invention relates to compositions comprised of a plastic component and a water-soluble hydroxylic polymer component and to a simple process for preparing these compositions. The hydroxylic polymer constitutes only a minor percentage, by weight, of the total composition and is present as a thin coating on the surface of the plastic. The hydroxylic polymer is typically a polysaccharide such as starch. Although the amount of polysaccharide deposited on the surface of the plastic is often too small to be seen without the aid of a microscope, it is sufficient to allow the hydrophobic plastic surface to be wet with a uniform film of water that does not readily separate from the surface as droplets or beads. Polysaccharide coatings are formed spontaneously, in the absence of any additives, when granular starch is dissolved at high temperature and pressure by steam jet cooking, and the resulting dilute starch solution is placed in contact with the plastic surface and allowed to cool. The polysaccharide coating remains attached to the plastic surface, even after prolonged soaking in water followed by drying.
2. Description of the Prior Art
The properties and chemical composition of starch and the methods used to prepare aqueous dispersions and solutions of starch are described in Eskins et al. (U.S. Pat. Nos. 5,676,994 and 5,882,713, which are herein incorporated by reference). Briefly, starch is a high polymer composed of repeating glucose units and is typically a mixture of linear and branched polymers, i.e., amylose and amylopectin. Cornstarch is the most plentiful and least expensive of all the commercial starch varieties. Although normal food grade cornstarch contains about 25% amylose by weight, commercial varieties of cornstarch are available that contain 0% amylose (waxy cornstarch) and about 50% and 70% amylose by weight (high amylose cornstarch). Normal cornstarch is the least expensive starch variety and is thus the preferred starch for the purposes of this invention. Starch occurs in living plants as granules ranging from about 5 to 40 microns in diameter, depending upon the plant source. These granules are essentially insoluble in water at room temperature; however, a significant amount of starch begins to dissolve and diffuse out of the granule matrix as the temperature reaches about 70° C., which is the approximate gelatinization temperature. Although water-solubility increases with temperature, starch granules do not dissolve completely, even at 100° C.; and a major portion of the starch remains as highly swollen but insoluble granule fragments. True solutions of starch in water, with no remaining insoluble material, are difficult to prepare using conventional batch-cooking techniques; and autoclaves are typically required for batch cooking. However, starch solutions are easily prepared on a continuous basis by passing aqueous dispersions of starch through a steam jet cooker at elevated temperatures and pressures. This process has been used commercially for decades to prepare starch solutions for industrial applications and is discussed in more detail in Eskins et al. (U.S. Pat. Nos. 5,676,994 and 5,882,713, supra) and also in an article by R. E. Klem and D. A. Brogly (Pulp & Paper, 55:98–103, May, 1981). Dissolved molecules of starch (especially amylose) tend to associate with one another through hydrogen bond formation and form gels and precipitates when solutions are cooled. This property is commonly known as retrogradation and is an inherent property of all starch pastes and solutions.
It is well known that practical end-use applications of polymeric materials are often dictated by their surface properties. This fact is particularly true for polymer films, for example, polyethylene. Although polyethylene films exhibit high levels of strength, toughness, flexibility and percent elongation, their hydrophobic surfaces repel water, thus ruling out or severely limiting their use in many applications. Adhesion, dye absorption, friction, electrostatic charging, biocompatibility and compatibility with hydrophilic reagents are all examples of polymer properties that are negatively influenced by poor surface wetting. A number of techniques for improving the surface wetting of hydrophobic polymers have been described, and the subject of surface modification of polymers has been summarized in an article by Dwight (CHEMTECH, March 1982, p. 166). Other articles have also appeared that describe methods used to modify polymer surfaces. For example, Rasmussen et al., (Journal of the American Chemical Society, Vol. 99, 1977, p. 4736) describe oxidation of the surface of polyethylene film with solutions of chromic acid and nitric acid. Foltynowicz (Macromolecules, Vol. 18, 1985, p. 1394) describes polymerization of surfactants on the surface of polyethylene to alter the surface properties. Dasgupta (Journal of Applied Polymer Science, Vol. 41, 1990, p. 233) describes the modification of polyethylene and polypropylene surfaces by treatment with ozone. Nakayama & Matsuda (Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 31, 1993, p. 3299) describe the fixation of poly (ethylene glycol) on the surface of hydrophobic polymers by chemical reaction. Terlingen et al. (Journal of Applied Polymer Science, Vol. 57, 1995, p. 969) describe the introduction of functional groups onto polyethylene surfaces by a carbon dioxide plasma treatment. Graft polymerization of hydrophilic monomers onto the surfaces of hydrophobic polymers is also a frequently used technique for modifying surface properties. Representative examples of this technique may be found in publications by Uchida et al. (Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 27, 1989, p. 527), Yamada et al. (Journal of Applied Polymer Science, Vol. 44, 1992, p. 993), and Tretinnikov & Ikada (Macromolecules, Vol. 30, 1997, p. 1086).
References related to the treatment of hydrophobic polymers to render their surfaces more hydrophilic are also found in the patent literature. For example, U.S. Pat. No. 4,728,694 discloses the graft polymerization of acrylamide onto polyethylene surfaces by first oxidizing the surface to form carbonyl substituents, reducing these substituents to form hydroxyl groups, and finally using a free radical initiator to initiate graft polymerization. In another representative example, U.S. Pat. No. 3,526,583 discloses a method for increasing surface hydrophilicity which involves the treatment of a hydrophobic polymer with a gas that has been excited by radio frequency radiation. Finally, U.S. Pat. No. 3,317,330 discloses a method for producing hydrophilic surfaces on polyethylene and polypropylene by treating the surfaces with an oxidizing solution that contains sulfuric acid, chromium trioxide and potassium permanganate.
It is apparent that in all of the references cited above, surface modification is carried out by treating hydrophobic polymers with chemical reagents that are either toxic, corrosive or expensive; and that the processing techniques employed are often complicated and costly. All of the above surface modification techniques are therefore unrelated to this invention, which provides a method for coating hydrophobic surfaces by a simple procedure that uses inexpensive and non-toxic starch.